Ultrafast free-space communications achieved with lasers

A new high-speed communications technology that is not limited by a
physical conductor, such as fiber optics, uses laser light to transmit data through
readily available open space. This optical free-space technology could revolutionize
communications, rendering environmental barriers meaningless and allowing mobile
units not tied to fiber optic cable to communicate in the range of 100 GHz and beyond,
the equivalent of 100 GB of data per second – which could lead to faster
Internet for the masses, supersensitive scanners and a more mobile military.

Dr. Rainer Martini of Stevens Institute of Technology and the
Ultrafast Laser Spectroscopy and Communication Laboratory hopes to extend the reach
into the terahertz spectrum; however, he and his team first need to face a fundamental
problem: optically induced modulation of lasers. A laser’s beam must be optically
modulated to transmit large amounts of data. Optically induced amplitude modulation
(AM) of mid-infrared lasers was realized by researchers at Stevens a few years ago,
but AM signals are at the mercy of dust and fog.

Left, Doctoral candidate Anderson Chen is pursuing his thesis in conjunction with Martini.
Now, Stevens researchers have developed a technique that optically
modulates the frequency of the laser beam as well (frequency modulation, or FM),
resulting in a signal that is disrupted significantly less by environmental factors.

Electronic modulation of a mid-infrared quantum cascade laser
is limited to 10 GHz, and optical modulation of frequency and amplitude offers a
viable alternative. Last year, Martini and his team developed a method to optically
induce fast amplitude modulation in a quantum cascade laser, a process that allows
them to control the laser’s intensity. Their system used a second laser to
modulate the amplitude of the mid-infrared laser – in essence, using light
to control light. But the researchers still faced the problem of reliability, so
they turned to optical frequency modulation.

Dr. Rainer Martini’s quantum cascade laser at the Ultrafast
Laser Spectroscopy and Communication Laboratory. Images courtesy of Stevens Institute
of Technology.
“FM-transmitted data is not affected by the environmental
elements that affect AM data,” Martini said. The recent success allows modulating
specifically the emission frequency of the laser – allowing a much more reliable
transmission. “But this was much more difficult to achieve and to prove.”

The optical approach has a number of applications, including frequency
modulation in a mid-infrared free-space communications system, wavelength conversion
that will transform a near-infrared signal directly into a mid-infrared signal,
and frequency modulation spectroscopy.